Hi,
I know there are folks here that keep shellies, but perhaps they keep them in species tanks so might not have your specific answer.
I don't have shellies, but certainly plan to try some at some point. In the meantime, I have 3 tanks with some Tangs. I have 2 15 gallons, each with what I think is a pair of Julidochromis Marlieri. I got them as a group of 4 and there is at least one pair among them that spawned for the previous owner, so I may need to put them all together again in a bigger tank if I don't see any spawning with my guessed pairs. My plan is to keep a pair in a 20 long eventually. Sexing them is tough. Anyway, from what I see of my Julies and others I've owned, I don't think you should have any trouble with putting them with shellies in a 75. They'll stay among your rock work and the shellies will stay with their shells.
In a 40 breeder I have a pair of Eretmodus, 7 Calvus (about 2 inches average) and a trio of Xenotilapia bathyphilus "Kekese". Based on what I see in the tank, the Calvus would probably work in the rocks as well, at least until they get big enough to eat the shellies, but you would loose a lot if not all shelly fry to them. The Eretomodus range all around and are a lot of fun, but probably too rough. The Xenos are really very gentle fish (for cichlids) and a lot of fun to watch sift through the sand. If you had a lot of sand areas remaining, there are several types of Xenos you might consider. They're not really territorial, but just roam around. Since they're mouth brooders it would give you a different spawning behavior to watch and you could strip some females if you wanted to keep fry.
To me, I think the shellies (maybe broken up into a few shell beds for interest), plus some Julies and something like xenos or possibly paracyprichromis to range around between would make a great 75.
Good luck and let us know how it goes.
